I had a hydra attack the party a few sessions ago while they were in canoes in an underwater cave. They were all 7th level but ran (rowed!) for their lives, it was going to be a TPK if they stayed, they had no way to effectively fight back if they got dumped in the jet black cold underwater river, it would have picked them off one by one.I did a CR 8 hydra last night, and the 5th level party stomped it.
Maybe I read it wrong, but fire damage stops all regeneration abilities - and fire damage is VERY common. And to even be able to regenerate a lost head, a head has to be chopped off (inflicting 25 points in a single turn).
So the party cut off two heads, but it was never able to regenerate due to fire damage.
I'm guessing a hydra dies at 0 hp even if it still has heads "alive?"
If they'd been on land, or able to use any kind of ranged attack or get away from it they would have destroyed it, but it was in its element and at a huge advantage. Even so, they (the PCs) didn't know to use fire, so it would have just kept growing heads.